Nonwoven fabrics composed of ultrafine carbon fibers have hitherto been widely used as impurity-removing filters and as fuel cell electrode components, including gas diffusion layers for fuel cells and electrode catalysts (see, for example, Patent Documents 1 to 7).
However, owing to the fact that the carbon fibers, which have an inherently low resistance to bending, have been made even finer in such nonwoven fabrics, the fabric is very brittle and lacks sufficient strength for processing. Accordingly, a drawback of nonwoven fabrics made of ultrafine carbon fibers is that they cannot be used alone to form such components.
To compensate for such a drawback and enable use in a variety of applications, it has been necessary to reinforce the fabric in some way, such as by increasing the thickness of the nonwoven fabric itself, forming a composite with larger-diameter carbon fibers that already exist, or bonding together the fibers with a binder.
However, applying such reinforcing treatment not only increases the thickness, it also gives rise to other problems which hinder use of the fabric, such as a loss of breathability.
Also, heating to at least 800° C. is generally required to carbonize organic compound, but most organic compound which serves as the carbon precursor has a glass transition point or melting point at or below 800° C. Therefore, when an ultrafine-fiber nonwoven fabric is heated, the organic fibers making up the fabric fuse or deform before the firing temperature is reached, making it impossible to maintain the shape of the fibers.
Hence, in the case of phenolic resins, melting during firing is prevented using a crosslinking agent such as formaldehyde to chemically effect three-dimensional crosslinking beforehand.
With resins such as polyacrylamide, infusibilizing treatment is generally carried out wherein the fibers are gradually heated in air (in the presence of oxygen) so as to oxidize the fiber surfaces and thereby form on the fiber surfaces an organization coat which does not melt. As a result, the fiber shape remains unchanged up to the firing temperature.
Firing and carbonizing ultrafine fibers in this way without associated shape deformation due to melting requires the formation of a three-dimensionally crosslinked structure (thermosetting or hardening) or infusibilization. Polymers that allow this to be done are limited to fibers capable of being infusibilized such as polyacrylonitrile and cellulose fibers, and thermosetting fibers such as amide and amide-imide fibers.
Moreover, infusibilizing ultrafine fibers without associated shape deformation has required strict temperature control.
High-strength ultrafine carbon fibers (carbon nanotubes, or “CNT”) are also known.
Yet, although CNTs are both ultrafine and high-strength, because the fibers are of short length, they cannot by themselves be rendered into a nonwoven fabric, and must be consolidated with a binder.
Another drawback is that CNT production requires complex operations.
A flexible carbon nanofiber has been reported in  Non-Patent Document 1 (Non-Patent Document 1). This is obtained by dissolving, in methanol as the solvent: a phenolic resin, high-molecular-weight polyvinyl butyral, and also, as electrolytes, pyridine and sodium carbonate (Na2CO3). The resulting solution is electrospun into a nanofiber nonwoven fabric, which is then subjected to crosslinking treatment with formaldehyde in a hydrochloric acid solution, neutralized and washed, then fired.
However, this production process is highly involved. Moreover, although the resulting carbon nanofibers do exhibit a certain degree of flexibility, they break when bent in two, and thus leave something to be desired in terms of flexibility.